Process of forming paper containing aluminum ions coordinated with hydroxide ions and ethylenically unsaturated aldehyde polymers and paper thereof



June 1967 s. P. MALCHICK ETAL 3,325,347

PROCESS OF FORMING PAPER CONTAINING ALUMINUM IONS COORDINATED WITH HYDRQXIDE IONS AND ETHYLENICALLY UNSATURATED ALDEHYDE POLYMERS AND PAPER THEREOF Filed Sept. 5, 1964 ALDEHYDE PO LYM ER PH Awusrma AGENT 3 r 1 I 1' PULP SUSPENSION MIXING CHAMBER 7 omu'rwm WATER DELUTIQN CHAMBER a PAPER Foammq DEVICE l V DRAINAGE DRYING 9 WATER DEV'CE REC-YCLE f WET AND DRY TENSILE STRENGTH PAPER Inventors Sherwin. P. Malchick William G. James John 3. Mundaa 3 Ma ga, dolmstorg k- 120013 Mtoraeg United States Patent Juliet, and John S. Munday, Nalco Chemical Company, Chicago, Ill., a corporation of Delaware Filed Sent. 3, 1964, Ser. No. 394,299 7 Claims. (Cl. 162-168) This invention relates to an improved paper process and resultant products having increased dry and wet tensile strength and other improved qualities. More particularly, this invention is concerned with a process of precipitating aluminum ions onto paper pulp fibers, such that the residual cationic charge of the precipitated aluminum ion is suflicient to retain an anionic polymer which is thus capable of increasing the wet and dry tensile strength of the paper.

Considerable effort has been expended to achieve high tensile strength, both wet and dry, through addition to paper of anionic polymers, but up until now none have attained a large measure of commercial success in the paper industry. An important failing of commercially available anionic Wet strength agents is inability to be retained by the paper pulp fibers without the expenditure of considerable expense and effort.

A wide variety of pulps are used in the papermaking art, some of which contain relatively large amounts of impurities while other pulps consist almost entirely of pure alpha cellulose fibers. Generally, the more refined pulps, such as alpha sulfite pulps, are the most difficult to treat with conventional wet strength agents. It would be of benefit to the art if a process for obtaining wet and dry tensile strength could be found which would be applicable to both the relatively impure pulps, such as groundwood, and the very pure pulps, such as alpha sulfite.

Because the paper pulp fibers themselves are anionic, it is necessary to have some types of bridging mechanism between the fiber and an anionic polymer in order to efiiciently retain the polymer. This bridging can sometimes be accomplished by cationic retention aids, such as polyamines and the like, but in most cases the additional expense of these cationic retention aids very often makes their use uneconomical. Also, the employment of conventional methods of retaining anionic materials in the papermaking art are also found to be limited. The use of alum, sodium aluminate, or aluminum chloride, although inexpensive, has proved to be unsatisfactory in many cases. In would, therefore, be of great advantage to the papermaking art if an improved method of retaining anionic polymers could be developed utilizing one or more of these just listed inorganic retention aids.

The traditional papermakers way of retaining anionic .materials is to put aluminum ions, such as in the form of aluminum sulfate, into the system, which then exhausts itself onto the fibers and makes the fibers cationic. While this practice is fine in theory, ineflicient and is commercially successful only because a number of anionic materials do not depend upon this mechanism for retention. Typical rosin sizes, for example, are retained by their being physically precipitated by the acid quality of alum, as well as because of the fact of a complex formed between the size particles and the sulphate ions.

However, until now, use of more common sources of aluminum ions has not been found to be successful in retaining anionic polymers, especially when the polymers this method is extremely the move pure paper pulps. The aluminum ions, at the pH range of the paper pulp employed in normal paper mill operations, are in equilibrium with part of the aluminum ions in solution and part insoluble and precipitated either on pulp fibers or suspended in the pulp slurry. At the lower pH range, such as at a pH of 4.5, where most of paper additives are particularly effective, a substantial portion of the aluminum ions are not directly associated with the pulp. Thus, aluminum ions in solution are competing with aluminum fixed on the paperfor the anionic materials being added. In such cases, only a fraction of the amount of anionic materials added is effectively retained. In papermaking processes that have recycling devices for the process water, an aluminum build-up in the system can cause a further decrease in the amount of anionic material retained.

It therefore becomes an object of this invention to provide a method of retaining anionic materials, and especially anionic water-soluble polymers on cellulosic fibers.

It is another object of this invention to provide a method for obtaining paper articles possessing high wet and dry tensile strength.

It is a further object of the invention to provide a paper article having permanent wet tensile strength. Other objects will appear hereinafter. It has now been discovered that the above and other objects may be accomplished by the process of this invention. Specifically, it has been found that aluminum ions may be added to a paper pulp suspension and subsequently treated in such a way to cause the pulp fibers to be a receptive agent in retaining anionic Water solubilized polymers. It has also been discovered that if alpha, beta-ethylenically unsaturated aldehyde polymers are used in the above paper application, unexpectedly high values of wet and dry tensile strength of paper materials are achieved. Further, the high wet strength realized is exceptionally resistant to degradation from exposure to water over long periods of time. This process, described in detail below, can be successfully utilized with employment of even the most pure pulps, and the same effectiveness is achieved with relatively pure pulps, as is realized with the more impure or easier to treat pulps.

The particular class of polymers which have been found are used as additives for most suitable as tensile strength increasing agents consists of anionic water-soluble polymers containing a substantial portion of an alpha, beta-ethylenically unsaturated aldehyde, such as acrolein or methacrolein. Preferred polymers of this type contain at least 30 mole percent and most preferably 50 mole percent of an alpha, beta-ethylenically unsaturated aldehyde.

Alpha, beta-ethylenically unsaturated aldehydes, as used herein, have an ethylenic group in an alpha-beta position relative to the aldehyde group. Examples of these compounds are acrolein and alpha and beta-substituted acroleins, such as alpha-ethylacrolein, beta-isobutylacrolein, alpha-chloroacrolein, beta-phenylacrolein, alpha-decylacrolein, alpha-cyclohexylacrolein, and the like. Preferred are acrolein and methacrolein.

The discussion and examples below which set forth the specific details of the process of this invention has been limited to polymers which are derived from acrolein. However, it is to be understod that any of the other alpha, beta-ethylenically unsaturated aldehydes are equally suitable for use in the process of this invention, and it is intended that these other aldehydes be included within its scope.

A wide variety of other monomers may be copolymerized with the alpha, beta-ethylenically unsaturated aldehydes set out above. Comonomers which are suitable may be broadly defined as any ethylenically unsaturated monomer which will copolymerize with alpha, beta-ethylenically unsaturated aldehydes as defined a'bove. Examples of suitable comonomers are acrylamide, acrylic acid, alkyl esters of acrylic acid such as methyl acrylate, etc., and salts of acrylic acid. Likewise, compounds such as maleic acid, and derivatives theerof, such as esters, salts, etc., vinyl sulfonic, and vinyl phosphonic acids, methyl isopropenyl ketone, trirnethylamine methacrylate, diethyl methylene succinate, ethyl vinyl ketone, vinyl acetate, vinyl pyrrolidone, alkyl'alcohol, sulfonated styrene, vinyl pyridine, sodium maleate, N-alkyl amines, and alkyl amines may be used.

The compounds which may be used in making copolymers or terpolymers useful in the process of this invention also include the ethylenically unsaturated carboxylic acids and their anhydrides such as methacrylic acid, crotonic acid, alpha-phenylacrylic acid, alpha-cyclohexylacrylic acid, beta-phenylacrylic acid, alpha-chloromaleic acid, tetrahydro. phthalic acid, methyl-tetrahydrophthalic acid, chlorornaleic acid, 7,9-dodecadienoic acid, 10,12-eicosadienoic acid, cyanoacrylic acid, methoxyacrylic acid, and the monobutyl ester of fumaric acid.

Other monomers which can be included in the anionic aldehyde polymers of the invention are styrene, acrylonitrile, methacrylonitrile, vinyl halides, monoolefins, and diolefins, such as ethylene, propylene, butylene, octylene, butadiene, isoprene, N-ethyl acrylamide, and the like.

It is preferred that any comonomer or monomers employed with the above described alpha, beta-ethylenically unsaturated aldehydes comprise no more than 70 mole percent of the polymer and most preferably no more than 50 mole percent of the polymer.

The term alpha, beta-ethylenically unsaturated aldehyde polymer, as used in this invention, includes the homopolymer of the aldehyde and also polymers derived from monomer mixtures of the aldehyde and at least one other ethylenically unsaturated addition type monomer as exemplified above, such that at least 30 mole percent of the monomer mixture is the aldehyde. For example, the term acrolein polymer includes the homopolymer as well as copolymers of acrolein and other monomers, terpolymers made from acrolein and two other monomers of the type listed above.

All of the above polymers may be conveniently prepared by conventional free radical initiation polymerization processes. Polymerization methods which yield relatively high molecular weight polymers are preferred. Generally, the aldehyde polymers are subsequently watersolubilized by addition of an alkali metal bisulfite or sulfur dioxide to form the bisulfite adduct. Sodium bisulfite is the preferred solubilizing agent.

The only specific requirement which must be placed on the alpha, beta-ethylenically unsaturated aldehyde polymers set out above is that said polymers must be capable of curing to a water-insoluble form when dried. Any of the above described polymers, when solubilized by the methods disclosed above, become substantially insoluble during the conventional drying processes employed in the paper industry.

The specific process details by which the above-described polymers are most efficiently retained are set out as follows. First, aluminum ions are added to a paper pulp suspension from an aluminum source wherein the aluminum ions exist in the +3 valence state. Once the aluminum ions are uniformly mixed throughout the paper pulp, they are precipitated by the addition of sufficient basic precipitating agent until the alumium ions have complexed with at least two but less than three hydroxyl ions per aluminum ion. At this time the acrolein polymer is added in an amount ranging from at least 0.05% to about by weight based on. the bone-dry fiber weight. The final step consists simply of forming the paper pulp suspension containing polymer into a fibrous Web and drying the treated paper.

In order to properly apply the aluminum ions, it is necessary that the compound source of an aluminum ion be essentially free from multivalent anions. Aluminum ions in an aqueous solution at a pH of 4.0 or less are unassociated with any anionic radicals and exist in a +3 valent state. As the pH is gradually increased to about 4.4, the aluminum ions will begin to coordinate with hydroxide ions in solution. If there are no interfering anions present, at a pH of about 4.4 each aluminum ion will be coordinated with one hydroxide ion. The presence of multivalent anions interferes with the hydroxide coordination, since multivalent anions such as sulfate and phosphate preferentially coordinate with the aluminum ions. If the pH is raised to about 4.6, two hydroxide ions coordinate with each aluminum ion and the aluminum will be uniformly precipitated onto the fibers of the paper pulp at this point. The resulting hydrated alumina precipitate has a residual cationic charge and will thereby attract and retain anionic particles. A further increase in basicity up to a pH of about 8.5 will result in a third hydroxide ion coordinating with the precipitated aluminum ion and neutralizing any remaining positive charge. It is preferred that the pH be maintained below this value, in order to preserve a residue of cationic charge. It is most preferred to maintain the pH below 6.0 for this reason.

It is therefore essential to select as a source of aluminum ions a compound which is free from multivalent anions such as sulfate ions which are found in alum. Consequently, aluinum chloride and the alkali metal aluminates are preferred, with aluminum chloride being most preferred as a source of aluminum ions.

The basic concepts of this invention are shown in the drawing. A pulp suspension 1 is added to a mixing chamber 5 and is mixed until a uniform suspension is obtained. In a normal paper mill operation, this mixing chamber might be the beater, the storage tanks, or any of the stock chests which are capable of suitably mixing pulp. Aluminum ions 2 are then added to the mixing chamber and mixed with the pulp suspension at a pH below 4.0 until the aluminum ions are uniformly distributed throughout the paper pulp suspension. In most cases, the source of aluminum ions will not be sufficiently acidic to lower the pH to below 4.0 and it will be necessary to add acid to assist in maintaining the pH below 4.0. When aluminum chloride is employed as an alumi num ion source, only minor additive amounts of acid may be necessary to adjust the pH to below 4.0 due to the acidic nature of aluminum chloride. In some instances, such as when 5% by weight or more of aluminum chloride is used, generally no additional acid is needed. However, this is dependent upon many factors such as local water pH and buifering. Other aluminum ionsources, such as sodium aluminate, require greater amounts of acid, depending again upon local water conditions. In all cases, it is essential that the acid used in adjusting the pH be free from multivalent anions. Simple mineral acids, such as hydrochloric acid and nitric acid, are preferred, while hydrochloric acid is most preferred.

At this point, either in the same mixing chamber or in another vessel, a pH adjusting agent 3 is added in an amount sufficient to precipitate the aluminum ion uniformly on to the pulp, such that the aluminum ion is coordinated wit-h at least two but less than three hydroxide ions. Any suitable basic pH adjusting agent may be used provided that the basic compound does not contain multivalent ions. The alkali metal hydroxides are preferred, and sodium hydroxide is most preferred because of its economy and abundancy.

It is most preferred that the aluminum ions be coordinated with at least 2.0 but less than 2.75 hydroxide ions, so that sufficient cationic charge will remain on the precipitated aluminum to attract the anionic acrolein polymer.

The resulting fiber-precipitated aluminum ions form an anion-receptive pulp due to the residual cationic charge The last step in this process simply involves the normal drying step. Any of the conventional apparatus used in the paper industry may be employed here. Normally, the paper is dried to less than 15% moisture, based on the of the precipitated aluminum ion. The amount of alumi- Weight of e p p- The h'y h -P 18 therefore a P num ion to be added is dependent on the amount of ferred emhedfment 0f the lhvehhonanionic polymer which is to be employed, and this in The fOlIOWlhg e p e e Presented to more clearly turn is dependent on the extent of improvement in tensile illustrate Concepts of lhls lhvenllon: strength desired. Usually, at least 0.1% and less than Example 1 30% of a source of aluminum ion, calculated as alumihum Chloride and based on the Weight of the fiber Solids Table 1 illustrates the effects of certain'variables in the of the Paper P p is added to Provide suffieieht P process of this invention. In Table 1, 2% by weight aluel'pita'ted aluminum retain the Subsequently added minum chloride was used as the source of aluminum ion, acrolein P y The Preferred Source of aluminum i011 and 1.25% of a sodium bisulfite-solubilized acrolein polyis aluminum chloride and the amount of aluminum ion mer was dd d, ba d o the Weight of the fibers in the may be conveniently calculated on the basis of aluminum pulp. The pulp used was an alpha sulfite pulp suspension ehlefide, regardless of the sour ee of aluminum i011- beaten to 100 sec. Williams freeness. Run No. 1 of Table The next p in the Process is the addition to the lwas made in the preferred manner; that is, the aluminum anion-receptive pulp of an anionic water-soluble aldehyde i was dd d to the pulp and dilute hydrochloric acid Polymer Satisfactory improvements in tensile Strength added to adjust the pH to 3.8; the pH was increased to 4.6 can be achieved y the addition of 0115% T0 100% of by the addition of sodium hydroxide to cause the aluacrolein p y based on the weight of the fibers in minum ion to precipitate onto the fiber by coordination the pulp. Addition of greater than 10% by Weight Of with at least 2.0 hydroxide ions; the acrolein polymer was acrolein polymer, though possible by the concepts of this added and the pulp suspension was formed into paper withinvention, tends to impart characteristics such as brittlet any f th r pH adjustment. Run No. 2 was the same ness to the paper, which property is not usually desir as Run No. 1, except that the pH of the system after the in conventional paper products- It is generally m s pr addition of the alumnium ion was 4.2 or higher at all f rr d to i dh 01% and 2% y weight 0f the times. This is in contradistinction to the requirement in acrolein polymer. the instant process that the pH be 4.0 or less after addi- Referring again t he draw g, the t u r t P 11 tion of aluminum ion. Run No. 3 was the same as Run having precipitated thereon aluminum ions and aldehyde No, 1 except that the polymer wa added before the H polymer attached to the aluminum ions, is transferred to was adjusted upward to precipitate the aluminum ion. a dilution chamber 6. At this point in a papermaking Again, such variant contravenes the teachings of the process, it is most common to dilute the pulp to a conpresent invention that polymer be added only after presistency of from 0.1% to 5% pulp, based on the weight cipitation of aluminum ions upon the fibers. Run No. 4 of the system. This dilution step is not an essential element was the same as Run No. 1 except that dilute aqueous hy-- of the invention, but in most papermaking operations a drochloric acid was added after the polymer addition in an dilution chamberis utilized. The water for the dilution amount sufiicient to decrease the pH to 4.3. As mentioned of the pulp suspension can either be fresh water 7 or above, addition of acid after polymer addition tends to drainage water 9 recycled from the paper-forming device. break down the polymer-fiber bond. Run No. 5 was a In either case, if dilution water is an employed variant, control. it is essential that it have a pH of at least as high as that As can be seen, Runs 2, 3, and 4am markedly less efiec of the treated 'pulp suspension prior to such dilution. tive in increasing the tensile strength of paper. In each of It has been found that the addition-of an acidic solution these three runs, one of the important features of this having a pH below that of the treated pulp to the alreadyinvention was omitted or deliberately varied beyond the formed aluminum precipitate will cause a decrease in scope of the present teachings. This demonstrates the necthe overall efiiciency of the process. essity of carrying out each step of the invention as de- Once the polymer has been applied to the pulp and scribed and in proper combination to achieve the superior any dilution has been efiected, the treated pulp suspenresults of Run No. 1.

TABLE 1 Run pH After pH at Polymer pH at Paper Dry Tensile Wet Tensile No Aluminum addition Formation Strength, Strength, Addition lbs/inch lbs/inch sion is added to a paper-forming device 8 which consists of any machine that drains water from an aqueous pulp suspension to form a solid pulp product. In most cases, this machine will be the typical Fourdrinier machine used in the paper industry. The paper-forming device, however, may be also more specialized, such as those used in the molded pulp industry, the paper board industry, and the like.

Example 2 The adverse effect of use of bivalent anions upon paper tensile strength is shown in Table 2. Paper was prepared from an alpha sulfite pulp which had been treated with aluminum chloride and a water-soluble sodium bisulfiteacrolein polymer as set out in Run No. l of Example 1 above. 1.25% of polymer was added, based on the weight of the pulp fibers. The only additive variant was the particular acid which was used to lower the starting pH of the aluminum ion-pulp suspension to 3.8.

TABLE 2.-EFFECT OSF 1ACID VALENCE ON TE'NSILE As can be seen from the table above, a significant reduction in paper strnegth occurs when bivalent anions,

such as sulfate ions, are introduced into the system. Particularly, wet strength is dramatically reduced. The same adverse effect is found when any bivalent anion is present. Particularly, when alum is used as a starting source of aluminum ion poor results are obtained, as noted in the table above.

Example 3 Examples of runs involving addition to pulp of a number of various polymers of acrolein are listed in Table 3. Paper was prepared using these polymers according to the process of this invention as described hereinabove. As can be seen, a wide variety of polymers are capable of providing substantial increases in tensile strength of paper products when used in the process of this invention.

TABIlE 3.ACROLEIN POLYMERS Monomer Dry Wet Acroleln Polymer Ratio- Tensile Tensile Mols' Strength, Strength,

lbs/inch lbs/inch Homopolymer 100:0 29. 5 9. 1 Acrolein-acrylonitrile. 67:33 29. 1 8.9 Acrolein-acrylic acid 75:25 26. 4 7. 8 Acrolein-acrylonitrile-acrylic acid. 72:24: 1 28. 4 8. 7 Acrolein-acrylonitrile-maleic anhydride 70; 25:5 28.0 9. 2

Ratio of acrolcimother listed monomers.

What is claimed is:

1. A process for the preparation of paper having increased wet and dry tensile strength and other improved properties which comprises the steps of:

(a) mixing with an aqueous paper pulp suspension at least 0.1% and less than 30% of a source of aluminum ions free from multivalent anions, calcualted as aluminum chloride and based on the bone-dry weight of the paper fiber, at a pH below 4.0 for sutficient time to obtain a uniform dispersion of said aluminum ions in said aqueous paper pulp;

(b) adjusting said pH of said dispersion upward by addition of a basic inorganic hydroxide until said aluminum ions precipitate onto the paper fiber and have coordinated therewith from at least 2.0 but less than 3.0 hydroxide ions, said resulting precipitated aluminum ions and paper fiber suspension thus having a cationic charge, and thereby forming an anion receptive pulp;

'(c) subsequentlyadding a water-solubleanionic alpha-,

beta-ethylenically unsaturated aldehyde polymer to said anion receptive pulp, said aldehyde polymer being capable of curing to an insoluble form and being added in an amount ranging from at least .05 to no more than 10% based on the bone-dry weight of said paper fiber;

(d) forming said aqueous suspension of said anion receptive pulp containing said polymer into a fibrous web, and drying said fibrous web for sufiicient time to cure said polymer.

2. The process of claim 1 where the pH of the mixture of aluminum ion and aqueous paper pulp dispersion is maintained below 4.0 by adding a mineral acid having only monovalent anions.

3. The process of claim 1 where the water-soluble alpha, beta-ethylenically unsaturated aldehyde polymer is selected from the group consisting of acrolein polymers and methacrolein polymers.

4. The process of claim 1 where the amount of alpha, beta-ethylenically unsaturated aldehyde polymer ranges from 0.1% to 2.0% based on the bone-dry fiber weight.

5. The process of claim 1 where, prior to the formation of said fibrous web from said aqueous suspension of anion receptive pulp containing said alpha, beta-ethylenically unsaturated aldehyde polymer, the alpha, beta-ethylenically unsaturated aldehyde polymer-treated pulp suspension is diluted with water having a pH at least as high as that of the treated pulp suspension prior to dilution, said water being added in an amount sufficient to give a consistency of from about 0.1% to 5% by weight of solids, based on the total weight of the paper pulp system.

6. A paper product comprising cellulosic fibers containing aluminum ions precipitated throughout said fibers, said aluminum ions being coordinated with at least 2.0 but less than 3.0 hydroxide ions and present in an amount ranging from at least 0.1% and less than 30%, calculated as aluminum chloride and based on the weight of said fibers, and a substantially water-insoluble alpha, betaethylenically unsaturated aldehyde polymer which is dispersed in and bonded to said fibers, the amount of said polymer present in said paper ranging from at least 0.05% to 10% by weight based on the bone-dry weight of said paper article.

7. The product of claim 6 where the alpha, beta-ethylenically unsaturated aldehyde polymer is selected from the'group consisting of acrolein polymers and methacrolein polymers.

References Cited UNITED STATES PATENTS 2,375,245 5/1945 Pretzel 162-182 X 3,074,843 1/ 1963 Lagally et a1 162-183 X 3,079,296 2/1963 Houff et al. 162-168 3,128,223 4/1964 Rosenberg et al. 162183 X FOREIGN PATENTS 3,810,353 6/1963 Japan.

S. LEON BASHORE, Acting Primary Examiner. 

6. A PAPER PRODUCT COMPRISING CELLULOSIC FIBERS CONTAINING ALUMINUM IONS PRECIPITATED THROUGHOUT SAID FIBERS, SAID ALUMINUM IONS BEING COORDINATED WITH AT LEAST 2.0 BUT LESS THAN 3.0 HYDROXIDE IONS AND PRESENT IN AN AMOUNT RANGING FROM AT LEAST 0.1% AND LESS THAN 3/%, CALCULATED AS ALUMINUM CHLORIDE AND BASED ON THE WEIGHT OF SAID FIBERS, AND A SUBSTANTIALLY WATER-INSOLUBLE ALPHA, BETAETHYLENICALLY UNSATURATED ALDEHYDE POLYMER WHICH IS DISPERSED IN AND BONDED TO SAID FIBERS, THE AMOUNT OF SAID POLYMER PRESENT IN SAID PAPER RANGING FROM AT LEAST 0.05% TO 10% BY WEIGHT BASED ON THE BONE-DRY WEIGHT OF SAID PAPER ARTICLE. 